Light and electron microscopic immunohistochemical methods, using specific (OX 6 and W3/25) monoclonal antibodies, were employed to investigate the changes that interstitial dendritic cells of the heart undergo in myocardial infarction induced by coronary ligation in Sprague- Dawley rats. The numbers of OX 6-positive interstitial dendritic cells/mm2 were 82 +/- 10 in the left ventricle of unoperated rats. Interstitial dendritic cells in the border zone of myocardial infarcts were markedly increased (796 +/- 79, 528 +/- 98, 159 +/- 15 at 7, 14 and 21 days, respectively), and were associated with clusters of T-helper lymphocytes. These findings suggest that interstitial dendritic cells participate in the activation of lymphocytes and in the initiation of immune responses in myocardial infarction.